Short-term multicycle combustion stimulation of oil wells



United States Patent 3,394,759 SHORT-TERM MULTICYCLE COMBUSTIONSTIMULATION OF OIL WELLS Kenneth Brandon Carey and Richard D. Goddard,Bakersfield, Califi, assignors to Esso Production Research Company NoDrawing. Filed Nov. 17, 1965, Ser. No. 508,375 6 Claims. (Cl. 16639)ABSTRACT OF THE DISCLOSURE A method of stimulating an oil-containingformation surrounding a well by a multicycle in situ combustiontechnique in which in each cycle a short period of burn is followed by ashort shut-in soak period. During the soak period the hot combustiongases and any unburned oxygen occupying the burned zone migrateups-tructure and the burned zone is resaturated with formation oil. Thewell is returned to production. Air injection is resumed while thereservoir temperature is sufiiciently high to ignite spontaneously thereservoir oil, and then the cycle is repeated.

The present invention concerns an improved method of well stimulation.More particularly, the present invention relates to improvements in oilwell stimulation by underground or in situ combustion.

Well stimulation by in situ combustion refers to a wellknown process inwhich a portion of the underground hydrocarbons in the reservoir orformation, particularly heavy viscous oils, are burned in place toprovide high temperatures and heat and thereby cause a reduction inviscosity of the formation hydrocarbon oil which is adjacent theburned-out interval or which enters the burnedout interval. In such aprocess an oxidizing gas, such as air or a mixture of air and oxygen orother gases capable of sustaining combustion of the formationhydrocarbons, is introduced into the subsurface formation through theproduction wellbore. The combustible mixture in the formation is ignitedin any desired manner, as by electric borehole heaters or chemicalcatalysts, such as phosphorous, triethylborane, or linseed oil. Theoxidizing gas is continuously supplied to the formation to maintaincombustion of the subsurface hydrocarbons. The amount of oxidizing gasinjected may range from 60 to 180 million cubic feet over a period ofone to three months. When the desired quantity of heat has beengenerated by such combustion, injection of the oxidizing gas isdiscontinued and burning ceases.

Prior to returning the well to production, a bottom hole choke isinstalled because of the hazardous combustible gas mixture (20 percentoxygen) initially produced from the formation. Thereafter, the well isimmediately returned to production of hydrocarbons through the bottomhole choke. After producing for several weeks or months until thereservoir pressure is no longer capable of sustaining production, a pumpand rods are run into the well to initiate artificial lift of the oilproduction. Then, when production diminishes or ceases and it is desiredto again stimulate the well, the aforementioned steps are repeated.

Although this conventional process successfully stimulates oilproduction, it has several limitations, the foremost of which is highoperating costs. The principal costs of operation are (1) preparation ofthe well for air injection; (2) use of a catalytic or other igniter; (3)compressor operation for 30 to 90 days of air injection; (4) bottom holechoke installation prior to fiowback; and (5) running pump and rods intoa well several weeks after burning to initiate artificial lift of theoil production.

3,394,759 Patented July 30, 1968 "ice A primary object of the presentinvention is to improve in situ operating procedure of the individualwell cyclic combustion stimulation process in order to attain (1)production stimulation similar in magnitude to that achieved by aconventional in situ procedure, such as described above; and (2)reduction in operating costs such that the overall operation isprofitable.

The process of the invention operates as follows. A conventional 30- to-day first cycle burn is initially conducted. In preparing the well fora second cycle burn, the rods and pump are pulled, and air (or otheroxidizing gas) injection is commenced. An ignition catalyst may berequired in this cycle. After injecting about 14 million cubic feet ofair into the well (e.g., seven days at 2.0M c.f./d.), the well isshut-in for another seven days. The shut-in period is known as the soakperiod. During this time, the hot combustion gases and any unburnedoxygen occupying the burned zone migrate up-structure and the burnedzone is resaturated with formation oil. Following the soak period, theWell is returned to production. It is produced for several Weeks ormonths until reservoir energy no longer sustains flowing production andwhile the reservoir temperature is sufficiently high to spontaneouslyignite upon resumption of air injection. Then, air injection is resumedand the entire cycle is repeated. Preferably, about eight wells areoperated in series, thereby making each complete individual well cycleof two-month duration.

Numerous advantages which result from the improved process include:

(1) Little well preparation is needed for the first short cycle burn andnone is required for subsequent cycle burns.

(2) Following the first short cycle burn, ambient reservoir temperatureis sufficient to initiate combustion simply upon resumption of airinjection, thereby eliminating the need for a supplemental ignitingcatalyst.

(3) Significantly less air injection is required per cycle to achievestimulation similar to that which results from the longer cycleconventional operation. The heat and pressure previously dissipated whenhot combustion gases and unburned oxygen were produced immediatelyfollowing termination of air injection are conserved in the reservoirwhen these gases migrate up-structure during the short (seven-day) soakperiod which follows air injection. This feature of the process tends toprovide greater and more lasting production stimulation of the well.

(4) The producing hazard created by initial production of a combustiblegas mixture containing hot hydrocarbons and about 20 percent oxygen,previously counteracted by running and producing through a bottom holechoke, is eliminated. Since in the improved process initial productionconsists of mostly liquid and since the small amount of gas producedcontains less than one percent oxygen, there is no need for theinstallation of a bottom hole choke during the early phases of fiowback.

(5) Since all production under the improved process is flowingproduction, the work and expense of running pumps and rods to place awell on artificial lift have been eliminated.

Additional benefits realized from the improved process are easiersurface handling and treating of the produced oil and an increasedgravity of the produced oil which makes it more valuable. These twobenefits result from thermal cracking of the crude hydrocarbons duringflow-back through the burned zone, which remains considerably hotter forthe short, more frequent burns than for longer, less frequent ones.

Thus, the process of the invention improves over previous in situcombustion techniques in including shorter and more frequent burn cyclesto sustain flowing production from the wells. Preferably, the burnperiod may be between four and ten days at an air injection rate of twomillion cubic feet per day. A short shut-in soak period follows the burnperiod. The length of the soak period is chosen sufficiently long toallow the burned zone to become resaturated by capillarity anddown-structure gravity drainage and to allow the combustion gases tomigrate up-structure out of the burned zone. Electrical or chemicalignition elements are eliminated (after the second cycle) because of thehigh reactivity of the native formation crude oil at the elevatedtemperatures. Ingition is attained spontaneously upon resumption of airinjection. An example of the cyclic operation in accordance with themethod of the present invention follows.

Well A was flowing and production was low. No work had been done on WellA since it had been burned previously. Well A was shut-in, theproduction line disconnected and the injection line connected. Noigniter Was necessary because the producing period since the previouscycle was short enough so that the formation temperature was stillsufliciently hot to ignite the formation crude oil spontaneously. Airwas injected for seven days at a rate of 2 million cubic feet per day.At the end of this period, air injection was switched to Well B and asoda ash solution was pumped into Well A for corrosion control. Well Awas then shut-in for seven days to soak, allowing the combustion gasesto migrate up-structure and the burned zone to resaturate withhydrocarbons. Then Well B was shut-in and Well A was opened forproduction. No bottom hole choke was run and liquid production wasobtained within two hours. Oxygen content of the produced gas was below1 percent in eight hours. Oil gravity was around 30 API, as compared tothe normal 18.

Having fully described the method, operation, objectives and advantagesof our invention, we claim:

1. A method of well stimulation comprising the steps of:

igniting formation hydrocarbons contained in a subsurface formationsurrounding a production well;

injecting an oxidizing gas into said formation through said well andburning a portion of the resident reservoir hydrocarbons to raise thetemperature of the reservoir rock remote from the wellbore; haltinginjection of said oxidizing gas; shutting-in said Well for a period oftime sufficiently long to allow the burned zone to become resaturated bycapillarity and down-structure gravity drainage and to allow thecombustion gases to migrate out of the burned zone; producing saidformation hydrocarbons through said production well so long as reservoirenergy sustains flowing production essentially in the liquid phase;

the length of said shut-in and production periods also being selectedsuch that upon resumption of the injection of said oxidizing gasfollowing termination of production, said formation hydrocarbonsremaining in said formation spontaneously ignite; and

then repeating the above steps of injecting oxidizing gas, shutting-insaid well and producing formation hydrocarbons.

2. A method as recited in claim 1 in which said oxidizing gas isinjected into said subsurface formation for from about four to ten daysat a rate of about 2.0 million cubic feet per day in each cycle.

3. A method of well stimulation as recited in claim 1 in which prior tothe first cycle recited therein, the following steps are carried out:

igniting said formation hydrocarbons and injecting oxidizing gas andburning a portion of said resident formation hydrocarbons to raise thetemperature of the reservoir rock remote from the wellbore, therebyreducing the viscosity of the remaining reservoir hydrocarbons;

halting injection of said oxidizing gas;

installing a bottom hole choke in said well;

producing said formation hydrocarbons through said production well untilreservoir energy no longer sustains flowing production;

running a pump and rods into said well and artificially lifting oilproduction;

halting artificial lifting of production; and

removing said pump and rods from said well.

4. A method of well stimulation comprising the steps of:'

igniting formation hydrocarbons contained in a subsurface formationsurrounding a production well and injecting oxidizing gas into saidformation through said well and burning a portion of said residentformation hydrocarbons to raise the temperature of the reservoir rockremote from the wellbore, thereby reducing the viscosity of theremaining hydrocarbons;

halting injection of said oxidizing gas;

installing a bottom hole choke in said well;

producing said formation hydrocarbons through said production well untilreservoir energy no longer sustains flowing production;

running a pump and rods into said well and artificially lifting oilproduct-ion;

halting artificial lifting of production;

removing said pump and rods from said well;

igniting said formation hydrocarbons;

injecting an oxidizing gas into said formation through said well andburning a portion of the resident hydrocarbons to raise the temperatureof the reservoir rock remote from the wellbore;

halting injection of said oxidizing gas;

shutting-in said well for a period of time sufficiently long to allowthe burned zone to become resaturated by capillarity and down-structuregravity drainage and to allow the combustion gases to migrate out of theburned zone;

producing said formation hydrocarbons through said production well solong as reservoir energy sustains flowing production;

the length of said shut-in and production periods also being selectedsuch that upon resumption of the injection of said oxidizing gasfollowing termination of production, said formation hydrocarbonsremaining in said formation spontaneously ignite; and

then repeating the above steps of injecting oxidizing gas, shutting-insaid well and producing formation hydrocarbons.

5. A method as recited in claim 4 in which said injection of saidoxidizing gas prior to the shutting-in of said well is continued forabout seven days at an injection rate of about 2.0 million cubic feetper day.

6. A method as recited in claim 5 in which said formation hydrocarbonsproduce-d following said shut-in period are produced essentially in theliquid state.

References Cited UNITED STATES PATENTS 3,332,482 7/1967 Trantham 16623,333,637 8/1967 Prats 166-2 X 3,129,757 4/1964 Sharp 166-11 3,139,9287/1964 Broussard 1662 3,172,472 3/1965 Smith 16638 3,179,169 4/1965Cline et a1. 166-38 3,180,412 4/1965 Bednarski et al. 166--11 3,259,1867/1966 Dietz 16611 3,266,569 8/1966 Sterrett 166-2 3,285,336 11/1966Gardner 166-11 STEPHEN J. NOVOSAD, Primary Examiner.

